This study attempts to elucidate the relationship between the structural and immunological properties of substructures of a complex protein molecule, Bovine Serum Albumin (BSA), and to provide information on how the immune system recognizes and responds to such molecule. Peptic fragments of BSA have been shown to be non-immunogenic and immunosuppresive. This immunosuppression was caused through activation of specific suppressor T-cells. Unique peptides of molecular weight 5-10,000 dalton have been identified and isolated following peptic digestion of BSA and they were found to possess immunomodulating properties. Cellular requirements for the immunomodulation induced by the peptides is being studied under in vitro conditions in cultures utilizing subpopulations of macrophages and lymphocytes.